Meaning: Falling off the chain.
Literally:
掉 = Fall/Lose/Reduce (only few of the many possible meaning)
链子 = Chain
Basically not going as planned.
You'll get the idea when you imagine this:
You're smoothly cycling on the road when the chain on your bike fall out. Your journey had been ruined.
Another alternative to remember the meaning easier is chain reaction. In a chain reaction, one sequence come after another. If one sequence doesn't appear, then the rest won't either.